Fig 3.
Model for the biogenesis of a-factor. The modules (CAAX processing, N-terminal cleavage, export, and receptor binding), biosynthetic intermediates (P0, P0*, P1, P2, MI, and ME), and enzymes (colored shapes) involved in the maturation of a-factor are indicated here and discussed in the text. Postprenylation CAAX modifications and the first step of N-terminal processing are carried out by integral membrane proteins (Rce1, Ste14, and Ste24) localized in the ER membrane, presumably with their active sites facing the cytosol. It is not known how a-factor is subsequently shuttled to the plasma membrane, where it is transported out of the cell by the ABC transporter protein Ste6. Extracellular a-factor binds to the receptor Ste3 on the cell surface of MATα cells. It should be noted that the cellular location of Axl1 and its site of a-factor processing have not been firmly established.